Valve operating mechanism with roller rocker arm, 4-cycle engine, and motorcycle having 4-cycle engine mounted thereon

ABSTRACT

A valve gear includes a camshaft having valve gear cams, first and second rocker shafts, a first rocker arm supported on the first rocker shaft, and a second rocker arm supported on the second rocker shaft: The valve gear cams, respectively, include a cam nose projecting from a base circle, and the first rocker shaft supporting the first rocker arm is positioned forwardly of a center line, which passes through a center of the camshaft to extend axially of a cylinder, in a direction of rotation of the camshaft:. The first rocker shaft is shifted closer to the camshaft than a center of rotation of a roller bearing is, when the roller bearing won the first rocker arm contacts with the base circle of the valve gear cam.

This Application is a National Stage application under U.S.C. § 371based on International Application No. PCT/JP2004/010879 filed on Jul.23, 2004, and further claims priority under 35 U.S.C. § 119 of JapaneseApplication No. 2003-207668, filed on Aug. 18, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a valve gear including rocker armshaving roller bearings at contact portions thereof with valve gear cams,and an overhead cam type four-cycle engine having such a valve gear.Further, the present invention relates to a vehicle such as a motorcycleincluding a four-cycle engine and such a valve gear.

2. Description of the Related Art

So-called SOHC (Single Overhead Camshaft) type four-cycle engines areknown and include a single camshaft that drives exhaust valves andintake valves. This type of four-cycle engine includes an exhaust rockerarm, which transmits movements of an exhaust cam to exhaust valves, andan intake rocker arm, which transmits movements of an intake cam tointake valves.

The exhaust rocker arm and the intake rocker arm, respectively, areswingably supported on rocker shafts. The rocker shafts are arranged inparallel to each other with a camshaft therebetween. Therefore, theexhaust rocker arm is located across the rocker shaft from the exhaustcam and extends toward the exhaust valves, and the intake rocker arm islocated across the rocker shaft from the intake cam and extends towardthe intake valves.

JP-B-07-068892 discloses a valve gear for four-cycle engines, in whichroller bearings are incorporated into an exhaust rocker arm and anintake rocker arm, respectively. The roller bearings come into rollingcontact with an exhaust cam and an intake cam to thereby restrict africtional resistance to a small amount, the frictional resistance beinggenerated at contact portions between the exhaust rocker arm and theexhaust cam and at contact portions between the intake rocker arm andthe intake cam.

In this conventional valve gear, when rotation of a camshaft causes theroller bearing of the intake rocker arm to contact a cam nose of theintake cam as it moves from a base circle thereof, the cam nose pushesup the roller bearing. As a result, the intake rocker arm swings on arocker shaft to push intake valves in an opening direction.

The rocker shaft, which supports the intake rocker arm, is positionedrearwardly of a centerline, which passes through a center of thecamshaft extending axially of a cylinder, in a direction of rotation ofthe camshaft. Therefore, when the cam nose pushes up the roller bearingof the intake rocker arm, the rocker shaft, which supports the intakerocker arm, is not moved in a direction in which the roller bearing ispushed up.

In other words, in a process in which the cam nose of the intake campushes up the roller bearing, a force exerted on a contact portion, atwhich the cam nose and the roller bearing contact with each other, actsin a direction intersecting a line which connects a center of rotationof the roller bearing and a center of the rocker shaft. Accordingly, aforce with which the cam nose pushes up the roller bearing acts as aforce by which the intake rocker arm is caused to swing on the rockershaft, so that any undesired force will not be applied to the intakerocker arm.

On the other hand, the rocker shaft, which supports the exhaust rockerarm, is positioned forwardly of a centerline, which passes through thecenter of the camshaft in the direction of rotation of the camshaft.Therefore, in a process in which the cam nose of the exhaust cam pushesup the roller bearing of the exhaust rocker arm, that rocker shaft whichsupports the exhaust rocker arm is moved in a direction in which theroller bearing is pushed up. Accordingly, a force exerted on a contactportion at which the cam nose and the roller bearing contact each other,acts along a line which connects a center of rotation of the rollerbearing and a center of the rocker shaft.

As a result, a force with which the cam nose pushes up the rollerbearing, acts as a force which causes the exhaust rocker shaft tobuckle, so that a load being applied on and borne by the exhaust rockerarm is increased.

Accordingly, it is necessary to take various measures to enable theexhaust rocker arm to handle such a buckling load. As a result, theexhaust rocker arm becomes disadvantageously heavy and large in size.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a valve gear that is capable ofpreventing a buckling load from being applied to a first rocker arm anddecreases a load being borne by the first rocker arm so that the firstrocker arm does not have to be reinforced and made larger.

Also, preferred embodiments of the present invention provide afour-cycle engine having such a novel valve gear, and a motorcycleinclude the engine having the novel valve gear.

A valve gear according to a first preferred embodiment of the presentinvention includes a camshaft having a first valve gear cam and a secondvalve gear cam, first and second rocker shafts arranged such that thecamshaft is disposed between the first and second rocker shafts, a firstrocker arm swingably supported on the first rocker shaft and having aroller bearing at one end thereof, the roller bearing of the firstrocker arm being arranged to contact with the first valve gear cam, anda second rocker arm swingably supported on the second rocker shaft andhaving a roller bearing at one end thereof, the roller bearing of thesecond rocker arm being arranged to contact with the second valve gearcam, wherein the first and second valve gear cams of the camshaft,respectively, include a base circle and a cam nose projecting from thebase circle, and the first rocker arm and the first valve gear cam arearranged such that when the roller bearing of the first rocker armcontacts with the base circle of the first valve gear cam, the firstrocker shaft is located closer to the camshaft than a location of acenter of rotation of the roller bearing of the first rocker arm.

With such a unique construction, when the camshaft rotates, the camnoses of the first and second valve gear cams push up the rollerbearings of the first and second rocker arms.

Since the cam nose is moved in a direction away from the second rockershaft during this process in which the cam nose of the second valve gearcam pushes up the roller bearing, the second rocker shaft will not bemoved in a direction in which the roller bearing is pushed up.Therefore, a force with which the cam nose pushes up the roller bearingacts as a force by which the second rocker arm is caused to swing on thesecond rocker shaft.

On the other hand, when the roller bearing of the first rocker armcontacts with the base circle of the first valve gear cam, the firstrocker shaft, which supports the first rocker arm, is located closer tothe camshaft than the location of the center of rotation of the rollerbearing. Therefore, the first rocker shaft will not be moved in adirection in which the roller bearing is pushed up during the process inwhich the cam nose of the first valve gear cam pushes up the rollerbearing.

Thus, the first rocker arm and the first valve gear cam are preferablyarranged such that a relationship between relative positions of a centerof the first rocker shaft, a center of rotation of the roller bearing ofthe first rocker arm, and a center of rotation of the camshaft is suchthat the first rocker arm does not buckle when the cam nose of the firstvalve gear cam contacts with the roller bearing of the first rocker armto cause the first rocker arm to swing in a valve opening direction

In addition, the first rocker arm and the first valve gear cam arepreferably arranged such that when the roller bearing of the firstrocker arm contacts with the base circle of the first valve gear cam, aforce exerted on a contact portion at which the cam nose and the rollerbearing of the first rocker arm contact each other does not act along aline which connects a center of rotation of the roller bearing of thefirst rocker shaft and a center of the first rocker shaft.

As a result of this unique construction, the force on the first rockerarm is greatly reduced and the first rocker arm does not need to bereinforced or enlarged.

It is preferred that the first rocker shaft is arranged to support thefirst rocker arm and is positioned forwardly of a center line whichpasses through a center of the camshaft to extend axially of a cylinderin a direction of rotation of the camshaft, and the second rocker shaftis arranged to support the second rocker arm and is positionedrearwardly of the center line in the direction of rotation of thecamshaft.

The roller bearing of the first rocker arm and the roller bearing of thesecond rocker arm, respectively, are preferably offset relative to thecenter line of the cylinder in an axial direction of the camshaft, andthe roller bearing of the first rocker arm offset farther from thecenter line than the roller bearing of the second rocker arm.

With the arrangement and movement described above, the first rocker armpreferably opens and closes at least one exhaust valve and the secondrocker arm preferably opens and closes at least one intake valve.

In a preferred embodiment of the present invention, the followingrelationship is satisfied: θ1>θ2, wherein θ1 indicates an intersectingangle between a line which connects the center of the first rocker shaftand the center of rotation of the roller bearing of the first rockerarm, and a line which connects a center of rotation of the camshaft andthe center of rotation of the roller bearing of the first rocker arm,and θ2 indicates an intersecting angle between a line which connects acenter of the second rocker shaft and a center of rotation of the rollerbearing of the second rocker arm, and a line which connects the centerof rotation of the camshaft and the center of rotation of the rollerbearing of the second rocker arm.

The intersecting angle θ1 is preferably larger than approximately 90degrees and the intersecting angle θ2 is preferably smaller thanapproximately 90 degrees.

In another preferred embodiment of the present invention, an engineincludes the valve gear according to the preferred embodiment describedabove. This engine is preferably a four-cycle engine.

According to yet another preferred embodiment of the present invention,a vehicle includes the engine described above having the valve gearaccording to the preferred embodiment described above, and the vehicleis preferably a motorcycle.

In a further preferred embodiment of the present invention, a four-cycleengine includes a cylinder having a bore center line, a cylinder headconnected to the cylinder and having an exhaust valve and an intakevalve, a camshaft supported by the cylinder head and having a firstvalve gear cam and a second valve gear cam, first and second rockershafts arranged such that the camshaft is disposed between the first andsecond rocker shafts, a first rocker arm swingably supported on thefirst rocker shaft and having a roller bearing at one end thereof, theroller bearing of the first rocker arm being arranged to contact withthe first valve gear cam, the first rocker arm acting to drive one ofthe exhaust valve and the intake valve, and a second rocker armswingably supported on the second rocker shaft and having a rollerbearing at one end thereof, the roller bearing of the second rocker armbeing arranged to contact with the second valve gear cam, the secondrocker arm acting to drive the other of the exhaust valve and the intakevalve, wherein the first and second valve gear cams of the camshaft,respectively, include a base circle and a cam nose projecting from thebase circle, and the first rocker arm and the first valve gear cam arearranged such that when the roller bearing of the first rocker armcontacts with the base circle of the first valve gear cam, the firstrocker shaft is located closer to the camshaft than a location of acenter of rotation of the roller bearing of the first rocker arm.

The exhaust valve and the intake valve, respectively, preferably includea valve stem at a first end thereof, and the first rocker arm and thesecond rocker arm, respectively, preferably include another end to pushthe valve stem, and the cylinder head includes a first opening to exposeabutting portions of the other end of the first rocker arm and the valvestem, and a second opening to expose abutting portions of the other endof the second rocker arm and the valve stem, the first opening and thesecond opening being arranged to be opposed to each other with the borecenter line therebetween and covered by respective common removablecovers, and the covers include first and second walls on inner surfacesthereof which are opposed to the abutting portions and arranged toreceive a lubricating oil, the first wall being formed with supply portsthrough which the lubricating oil is applied to the abutting portions ofthe other end of the first rocker arm and the valve stem, and the secondwall being formed with supply ports through which the lubricating oil isapplied to the abutting portions of the other end of the second rockerarm and the valve stem.

Other features, elements, characteristics and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing a motorcycle according to a preferredembodiment of the invention.

FIG. 2 is a cross-sectional view showing the positional relationshipamong a camshaft, an exhaust rocker arm, and an intake rocker arm in afour-cycle engine according to a preferred embodiment of the invention.

FIG. 3 is a cross sectional view taken along the line F3—F3 in FIG. 2.

FIG. 4 is a plan view showing the positional relationship between theexhaust rocker arm and the intake rocker arm in a four-cycle engineaccording to a preferred embodiment of the invention.

FIG. 5 is a plan view showing tappet covers according to a preferredembodiment of the invention.

FIG. 6 is a cross-sectional view showing a state, in which a rollerbearing of the exhaust rocker arm is pushed up by a cam nose of anexhaust cam, in the embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the drawings.

FIG. 1 shows a motorcycle 100 according to a preferred embodiment of theinvention. The motorcycle 100 preferably includes a frame 101. The frame101 includes a head pipe 102, left and right main frames 103 (only oneof them being shown), and left and right rear-arm brackets 104 (only oneof them being shown).

The head pipe 102 is positioned at a front end of the frame 101 tosupport a front wheel 106 through a front fork 105. The main frames 103extend rearwardly of the head pipe 102 and incline downward as theyextend rearwardly of the head pipe 102. The main frames 103 support afuel tank 107.

The rear-arm brackets 104 project downward from rear ends of the mainframes 103. The rear-arm brackets 104 pivotally support a rear arm 108.The rear arm 108 extends rearwardly of the rear-arm brackets 104. A rearwheel 109 is supported at a rear end of the rear arm 108.

Left and right seat rails 110 (only one of them being shown) areconnected to upper ends of the rear-arm brackets 104. The seat rails 110pass above the rear wheel 109 to extend rearwardly of the rear-armbrackets 104. The seat rails 110 support a seat 111. The seat 111 isconnected at its front end to the fuel tank 107.

As shown in FIG. 1, the frame 101 supports a water-cooled four-cyclesingle cylinder engine 1, for example, which drives the rear wheel 109.The engine 1 is disposed below the main frames 103 and supported by themain frames 103 and the rear-arm brackets 104.

As shown in FIGS. 1 and 2, the engine 1 preferably includes a crankcase1 a, a cylinder block 2, and a cylinder head 3. The crankcase 1 aaccommodates therein a crank shaft (not shown). The cylinder block 2stands upright from an upper surface of the crankcase 1 a. The cylinderblock 2 includes a cylinder 4. The cylinder 4 accommodates therein apiston 5. The piston 5 is connected through a connecting rod 6 to thecrank shaft.

The cylinder head 3 is connected to an upper surface of the cylinderblock 2. The cylinder head 3 includes a recess 7 located on a surfacethereof that is opposed to the cylinder 4. The recess 7 defines a pentroof type combustion chamber 8 between the recess 7 and a top surface ofthe piston 5.

As shown in FIGS. 2 and 4, the cylinder head 3 includes a pair ofexhaust ports 10 a, 10 b and a pair of intake ports 11 a, 11 b. Theexhaust ports 10 a, 10 b and the intake ports 11 a, 11 b are opened tothe combustion chamber 8 and arranged to be opposed to each other with abore center line O1 of the cylinder 4, which passes through a center ofthe combustion chamber 8, extending therebetween.

The cylinder head 3 supports two exhaust valves 12, which open and closethe exhaust ports 10 a, 10 b, and two intake valves 13, which open andclose the intake ports 11 a, 11 b. Valve stems 12 a of the exhaustvalves 12 are arranged substantially parallel to each other and inclinedin a direction such that portions of the valve stems 12 a are locatedfarther away from the bore center line O1 as the portions of the valvestems 12 a become more distant from the combustion chamber 8. Valvestems 13 a of the intake valves 13 are arranged substantially parallelto each other and inclined in an opposite direction to that of the valvestems 12 a of the exhaust valves 12 relative to the bore center line O1.

The valve stems 12 a of the exhaust valves 12 are shorter in totallength than the valve stems 13 a of the intake valves 13. Therefore, tipends of the valve stems 12 a are positioned below tip ends of the valvestems 13 a.

The exhaust valves 12 are biased in a direction in which the exhaustports 10 a and 10 b are closed by valve springs 14. Likewise, the intakevalves 13 are biased in a direction in which the intake ports 11 a and11 b are closed by valve springs 15.

As shown in FIGS. 2 and 3, a valve gear chamber 16 is formed within thecylinder head 3. The valve gear chamber 16 is positioned just above thecombustion chamber 8. Tip ends of the valve stems 12 a of the exhaustvalves 12 and tip ends of the valve stems 13 a of the intake valves 13project into the valve gear chamber 16.

The valve gear chamber 16 accommodates therein a valve gear 17 thatdrives the exhaust valves 12 and the intake valves 13. The valve gear 17includes a single camshaft 18, an exhaust rocker arm 19 defining a firstrocker arm, and an intake rocker arm 20 defining a second rocker arm.

The camshaft 18 is supported at its one end and its other end throughbearings 21 by the cylinder head 3. A center X1 of rotation of thecamshaft 18 is substantially perpendicular to the bore center line O1.One end of the camshaft 18 is positioned in the vicinity of the borecenter line O1. Therefore, the camshaft 18 is offset radially of thecylinder 4 relative to the bore center line O1.

The other end of the camshaft 18 is disposed outside the valve gearchamber 16. A sprocket 22 is fixed to the other end of the camshaft 18.A cam chain 23 is stretched between the sprocket 22 and the crank shaft.According to the present preferred embodiment, the camshaft 18 rotatesforward in a counterclockwise direction (a direction of rotation of thefront wheel 106 when the motorcycle 100 advances) indicated by an arrowin FIG. 2.

As shown in FIG. 3, the camshaft 18 includes an exhaust cam 25 defininga first valve gear cam, and an intake cam 26 defining a second valvegear cam. The exhaust cam 25 and the intake cam 26 are aligned axiallyrelative to the camshaft 18. When the cylinder head 3 is viewed axiallyof the cylinder 4, the exhaust cam 25 and the intake cam 26 are offsetaxially of the camshaft 18 relative to the bore center line O1. Anamount L1 of offset of the exhaust cam 25 relative to the bore centerline O1 is larger than an amount L2 of offset of the intake cam 26relative to the bore center line O1.

As shown in FIG. 2, the exhaust cam 25 includes a base circle 27 a,which maintains the exhaust valves 12 in a closed state, and a cam nose27 b, which actuates the exhaust valves 12 in a direction of opening.The cam nose 27 b extends beyond the base circle 27 a.

Likewise, the intake cam 26 includes a base circle 28 a, which maintainsthe intake valves 13 in a closed state, and a cam nose 28 b, whichactuates the intake valves 13 in a direction of opening. The cam nose 28b extends beyond the base circle 28 a.

As shown in FIGS. 3 and 6, oil jet ports 29 a, 29 b, respectively, areformed in the exhaust cam 25 and the intake cam 26. The oil jet port 29a is opened to an outer peripheral surface of the base circle 27 a ofthe exhaust cam 25. The oil jet port 29 b is opened to an outerperipheral surface of the base circle 28 a of the intake cam 26. The oiljet ports 29 a, 29 b are arranged to supply a lubricating oil torespective parts of the valve gear 17. Therefore, the lubricating oilthat is pressurized by an oil pump is applied on a circumference of thecamshaft 18 through the oil jet ports 29 a, 29 b.

As shown in FIGS. 2 and 4, the exhaust rocker arm 19 is swingablysupported by a first rocker shaft 30 on the cylinder head 3. The firstrocker shaft 30 is substantially parallel to and disposed above thecamshaft 18. Further, the first rocker shaft 30 is positioned forwardlyof the bore center line O1 which passes through the center X1 ofrotation of the camshaft 18, in a direction of rotation of the camshaft18. In other words, the first rocker shaft 30 is positioned between thecamshaft 18 and the valve stems 12 a of the exhaust valves 12.

The exhaust rocker arm 19 includes a substantially cylindrical-shapedboss 31, a roller support 32, and a pair of push arms 33 a, 33 b. Theboss 31 is swingably supported on the first rocker shaft 30. The boss 31is offset on one side in an axial direction of the first rocker shaft 30relative to the bore center line O1.

The roller support 32 is arranged to bifurcate and to project toward theexhaust cam 25 from an outer peripheral surface of the boss 31 Theroller support 32 rotatably supports a roller bearing 34. The rollerbearing 34 is positioned at one end of the exhaust rocker arm 19 to comeinto rolling contact with the base circle 27 a and the cam nose 27 b ofthe exhaust cam 25. A center X2 of rotation of the roller bearing 34 isoffset toward the first rocker shaft 30 relative to the bore center lineO1, which passes through the center X1 of rotation of the camshaft 18.

As shown in FIG. 4, the push arms 33 a, 33 b project toward the valvestems 12 a of the exhaust valves 12 from the outer peripheral surface ofthe boss 31. The two valve stems 12 a are evenly distributed with thebore center line O1 therebetween. In contrast, the boss 31 is offset onone side in the axial direction of the first rocker shaft 30 relative tothe bore center line O1. Therefore, one 33 a of the push arms and theother 33 b of the push arms are different in length from each other. Theother 33 b of the push arms, which pushes the valve stem 12 a disposedaway from the boss 31, crosses a line A, which extends radially of thecombustion chamber 8 so as to be substantially perpendicular to the borecenter line O1 and the camshaft 18.

Projecting ends of the push arms 33 a, 33 b are positioned at the otherend of the exhaust rocker arm 19 and opposed to the tip ends of thevalve stems 12 a, as seen in FIG. 6. Adjust screws 35, respectively, arescrewed into the projecting ends of the push arms 33 a, 33 b. The adjustscrews 35 abut against the tip ends of the valve stems 12 a.Accordingly, the exhaust rocker arm 19 extends across the first rockershaft 30 from the exhaust cam 25 to the tip ends of the valve stems 12 aof the exhaust valve 12.

Further, a pair of oil supply ports 36 are formed on the boss 31 of theexhaust rocker arm 19 as shown in FIG. 4. The oil supply ports 36receive the lubricating oil applied from the oil jet ports 29 a, 29 b ofthe camshaft 18 to apply the oil between the boss 31 and the firstrocker shaft 30. The oil supply ports 36 are spaced away from each otherin an axial direction of the boss 31.

As shown in FIGS. 2 and 4, the intake rocker arm 20 is swingablysupported on the cylinder head 3 through a second rocker shaft 38. Thesecond rocker shaft 38 is substantially parallel to and disposed abovethe camshaft 18. The second rocker shaft 38 is positioned rearwardly ofthe bore center line O1, which passes through the center X1 of rotationof the camshaft 18, in the direction of rotation of the camshaft 18.Therefore, the first rocker shaft 30 and the second rocker shaft 38 arearranged substantially parallel to each other with the camshaft 18located therebetween.

The intake rocker arm 20 includes a substantially cylindrical-shapedboss 39, a roller support 40, and a pair of push arms 41 a, 41 b. Theboss 39 is swingably supported on the second rocker shaft 38. The boss39 is offset on one side in an axial direction of the second rockershaft 38 relative to the bore center line O1.

The roller support 40 is arranged to bifurcate and to project toward theintake cam 26 of the camshaft 18 from an outer peripheral surface of theboss 39. The roller support 40 supports a roller bearing 42. The rollerbearing 42 is positioned at one end of the intake rocker arm 20 so as tocome into rolling contact with the base circle 28 a and the cam nose 28b of the intake cam 26. A center X3 of rotation of the roller bearing 42is offset toward the second rocker shaft 38 relative to the bore centerline O1, which passes through the center X1 of rotation of the camshaft18.

As shown in FIGS. 2 and 4, the push arms 41 a, 41 b project toward thevalve stems 13 a of the intake valves 13 from an outer peripheralsurface of the boss 39. The two valve stems 13 a are evenly distributedwith the bore center line O1 being disposed therebetween.

In contrast, the boss 39 is offset on one side in the axial direction ofthe second rocker shaft 38 relative to the bore center line O1.Therefore, one 41 a of the push arms and the other 41 b of the push armsare different in length from each other. The other 41 b of the pusharms, which pushes the valve stem 13 a disposed away from the boss 39,crosses the line A. Further, a space D1 between projecting ends of thepush arms 41 a, 41 b is larger than a space D2 between the projectingends of the push arms 33 a, 33 b of the exhaust rocker arm 19.

The projecting ends of the push arms 41 a, 41 b are positioned at theother end of the intake rocker arm 20 and opposed to the tip ends of thevalve stems 13 a. Adjust screws 43, respectively, are screwed into theprojecting ends of the push arms 41 a, 41 b. The adjust screws 43 abutagainst the tip ends of the valve stems 13 a. Accordingly, the intakerocker arm 20 extends across the second rocker shaft 38 from the intakecam 26 to the tip ends of the valve stems 13 a of the intake valve 13.

A pair of oil supply ports 44 are formed on the boss 39 of the intakerocker arm 20. The oil supply ports 44 receive the lubricating oil fromthe oil jet ports 29 a, 29 b of the camshaft 18 and apply the oilbetween the boss 39 and the second rocker shaft 38. The oil supply ports44 are spaced away from each other in an axial direction of the boss 39.

As shown in FIG. 2, when the roller bearing 34 on the exhaust rocker arm19 contacts with the base circle 27 a of the exhaust cam 25, the firstrocker shaft 30, which supports the exhaust rocker arm 19, is movedcloser to the camshaft 18 along the bore center line O1 of the cylinder4 than the center X2 of rotation of the roller bearing 34 is. In otherwords, a center X4 of the first rocker shaft 30 is disposed in a lowerposition than the center X2 of rotation of the roller bearing 34 as longas the roller bearing 34 contacts with the base circle 27 a.

Based on this, an intersecting angle θ1 is set to, for example,approximately 92° where θ1 indicates an intersecting angle between aline B1, which connects between the center X4 of the first rocker shaft30 and the center X2 of rotation of the roller bearing 34, and a lineB2, which connects between the center X1 of rotation of the camshaft 18and the center X2 of rotation of the roller bearing 34.

When the roller bearing 42 on the intake rocker arm 20 contacts with thebase circle 28 a of the intake cam 26, the second rocker shaft 38, whichsupports the intake rocker arm 20, is moved farther from the camshaft 18than the center X3 of rotation of the roller bearing 42 is. In otherwords, a center X5 of the second rocker shaft 38 is disposed in a higherposition than the center X3 of rotation of the roller bearing 42 as longas the roller bearing 42 contacts with the base circle 28 a.

Therefore, an intersecting angle θ2 is set to, for example,approximately 76° where θ2 indicates an intersecting angle between aline C1, which connects between the center X5 of the second rocker shaft38 and the center X3 of rotation of the roller bearing 42, and a lineC2, which connects between the center X1 of rotation of the camshaft 18and the center X3 of rotation of the roller bearing 42.

Accordingly, the intersecting angle θ1 is larger than the intersectingangle θ2 (θ1>θ2).

As shown in FIG. 3, the cylinder head 3 includes a recess 46, whichcaves toward the center of the combustion chamber 8. The recess 46 ispositioned in opposition to the camshaft 18 with the bore center line O1disposed therebetween. A plug mount hole 47 is formed at a bottom of therecess 46 to be opened to the center of the combustion chamber 8. Anignition plug 48 is screwed into the plug mount hole 47. An insulatingmaterial 48 a of the ignition plug 48 is positioned in the recess 46.

With such a cylinder head 3, the camshaft 18 is offset radially of thecylinder 4 relative to the bore center line O1. Therefore, a large spacefor formation of the recess 46 can be ensured in that portion of thecylinder head 3 which is opposed to the camshaft 18 with the bore centerline C1 disposed therebetween. As a result, the recess 46 can be locatedclose to the bore center line O1, so that it is possible to have theignition plug 48 standing upright relative to the combustion chamber 8.

As shown in FIGS. 2 and 4, the cylinder head 3 includes a first opening50 and a second opening 51, which are opened to the valve gear chamber16. The first opening 50 allows for tappet adjustment of the exhaustvalves 12, and is shaped in a manner to expose abutting portions of thevalve stems 12 a of the exhaust valves 12 and the push arms 33 a, 33 bof the exhaust rocker arm 19. The first opening 50 is positioned at afront end of the cylinder head 3.

The second opening 51 allows for tappet adjustment of the intake valves13, and is shaped in a manner to expose abutting portions of the valvestems 13 a of the intake valves 13 and the push arms 41 a, 41 b of theintake rocker arm 20. The second opening 51 is positioned at a rear endof the cylinder head 3. The first and second openings 50, 51 preferablyhave substantially the same shape as each other.

As shown in FIG. 2, the first and second openings 50, 51, respectively,are preferably covered by tappet covers 52. The tappet cover 52, whichcovers the first opening 50, and the tappet cover 52, which covers thesecond opening 51, are common to each other and fixed to the cylinderhead 3 in a removable manner.

The tappet covers 52 include an inner surface exposed to the valve gearchamber 16. First and second walls 54, 55 are formed on the innersurfaces of the tappet covers 52. The first and second walls 54, 55project obliquely downward toward the valve gear chamber 16 so as toreceive the lubricating oil applied from the oil jet ports 29 a, 29 b ofthe camshaft 18. The first and second walls 54, 55 are aligned andspaced at intervals along a height direction of the cylinder head 3, andextend axially of the first and second rocker shafts 30, 38.

As shown in FIG. 5, the first wall 54 is positioned above the secondwall 55. The first wall 54 preferably includes a pair of V-shaped oilguides 56 a, 56 b. The oil guides 56 a, 56 b include supply ports 57 a,57 b, which are arranged to extend in a width direction of the tappetcover 52 and are arranged to drip the lubricating oil, which is receivedby the first wall 54. A space D3 between the supply ports 57 a, 57 bcorresponds to the space D2 between the push arms 33 a, 33 b of theexhaust rocker arm 19.

The second wall 55 positioned below the first wall 54 includes a pair ofV-shaped oil guides 58 a, 58 b. The oil guides 58 a, 58 b include supplyports 59 a, 59 b, which are arranged to extend in a width direction ofthe tappet cover 52 and are arranged to drip the lubricating oil, whichis received by the second wall 55. A space D4 provided between thesupply ports 59 a, 59 b corresponds to the space D1 between the pusharms 41 a, 41 b of the intake rocker arm 20.

As a result of this unique structure, the supply ports 57 a, 57 b of thefirst wall 54 are positioned just above the projecting ends of the pusharms 33 a, 33 b of the exhaust rocker arm 19 in a state in which thefirst opening 50 on an exhaust side is covered by the tappet cover 52.Accordingly, the lubricating oil is supplied through the supply ports 57a, 57 b to abutting portions of the adjust screws 35 and the valve stems12 a of the exhaust valves 12.

Likewise, the supply ports 59 a, 59 b of the second wall 55 arepositioned just above the projecting ends of the push arms 41 a, 41 b ofthe intake rocker arm 20 in a state in which the second opening 51 on anintake side is covered by the tappet cover 52. Accordingly, thelubricating oil is supplied through the supply ports 59 a, 59 b toabutting portions of the adjust screws 43 and the valve stems 13 a ofthe intake valves 13.

Accordingly, although the first opening 50 on the exhaust side and thesecond opening 51 on the intake side are covered by the common tappetcovers 52, it is possible to surely supply the lubricating oil to theabutting portions of the adjust screws 35 and the exhaust valves 12 andthe abutting portions of the adjust screws 43 and the intake valves 13.

In particular, according to the present preferred embodiment, the boss31 of the exhaust rocker arm 19 and the boss 39 of the intake rocker arm20 are offset relative to the bore center line O1 axially of the firstand second rocker shafts 30, 38. Therefore, the oil jet ports 29 a, 29 bof the camshaft 18, from which the lubricating oil is applied, aredistant from the valve stem 12 a of the other of the exhaust valves 12and the valve stem 13 a of the other of the intake valves 13. As aresult, the lubricating conditions of the other of the exhaust valves 12and the other of the intake valves 13 become strict at the time ofidling operation, at which the lubricating oil is applied in smallamounts.

With the above constitution, the lubricating oil can be supplied to theabutting portions of the adjust screws 35 and the other of the exhaustvalves 12 and the abutting portions of the adjust screws 43 and theother of the intake valves 13 from the supply ports 57 a, 57 b, 59 a, 59b formed on the tappet covers 52. Therefore, even when the abuttingportions are distant from the oil jet ports 29 a, 29 b, the amount oflubricating oil being supplied to the abutting portions will not besmall. Accordingly, the reliability of the lubrication is greatlyimproved.

Subsequently, an operation of the valve gear 17 will be described withreference to FIG. 6.

FIG. 2 shows a state in which the roller bearing 34 on the exhaustrocker arm 19 and the roller bearing 42 on the intake rocker arm 20,respectively, contact with the base circle 27 a of the exhaust cam 25and the base circle 28 a of the intake cam 26. At this time, the exhaustvalves 12 and the intake valves 13 are closed.

When the camshaft 18 rotates forward in a counter clockwise directionindicated by the arrow in FIG. 2, the roller bearing 34 on the exhaustrocker arm 19 contacts and rides on the cam nose 27 b as it moves fromthe base circle 27 a of the exhaust cam 25. The cam nose 27 b pushes upthe roller bearing 34 of the exhaust rocker arm 19. Therefore, theexhaust rocker arm 19 swings on the first rocker shaft 30, and the pusharms 33 a, 33 b of the exhaust rocker arm 19 push down the valve stems12 a of the exhaust valves 12. Accordingly, the exhaust valves 12 areopened.

Subsequently, the roller bearing 42 on the intake rocker arm 20 contactsand rides on the cam nose 28 b as it moves from the base circle 28 a ofthe intake cam 26. The cam nose 28 b pushes up the roller bearing 42 ofthe intake rocker arm 20. Therefore, the intake rocker arm 20 swings onthe second rocker shaft 38, and the push arms 41 a, 41 b of the intakerocker arm 20 push down the valve stems 13 a of the intake valves 13.Accordingly, the intake valves 13 are opened.

The second rocker shaft 38, which supports the intake rocker arm 20, ispositioned rearwardly of the bore center line O1, which passes throughthe center X1 of rotation of the camshaft 18, in the direction ofrotation of the camshaft 18. Therefore, the cam nose 28 b of the intakecam 26 is moved away from the second rocker shaft 38 during the processin which the cam nose 28 b pushes up the roller bearing 42.

Accordingly, the second rocker shaft 38 will not be moved in a directionin which the roller bearing 42 is pushed up. As a result, a force withwhich the cam nose 28 b pushes up the roller bearing 42 acts as a forceby which the intake rocker arm 20 is caused to swing on the secondrocker shaft 38.

On the other hand, the first rocker shaft 30, which supports the exhaustrocker arm 19, is positioned forwardly of the bore center line O1, whichpasses through the center X1 of rotation of the camshaft 18, in thedirection of rotation of the camshaft 18. The first rocker shaft 30 isdisposed in a lower position than the center X2 of rotation of theroller bearing 34 when the roller bearing 34 of the exhaust rocker arm19 contacts with the base circle 27 a of the exhaust cam 25.

By virtue of this, the first rocker shaft 30 will not be moved in adirection in which the roller bearing 34 is pushed up during the processin which the cam nose 27 b of the exhaust cam 25 pushes up the rollerbearing 34. Accordingly, a force F exerted on a contact portion at whichthe cam nose 27 b and the roller bearing 34 contact with each other,acts in a different direction from that of the line B1, which connectsbetween the center X2 of rotation of the roller bearing 34 and thecenter X4 of the first rocker shaft 30, as shown by an arrow in FIG. 6.

In other words, the valve gear 17 determines and controls therelationship of relative positions of the center X4 of the first rockershaft 30, the center X2 of rotation of the roller bearing 34 of theexhaust rocker arm 19, and the center X1 of rotation of the camshaft 18such that the exhaust rocker arm 19 does not buckle when the exhaustrocker arm 19 swings in a direction in which the exhaust valves 12 isopened.

Although the first rocker shaft 30 is positioned forwardly of the borecenter line O1 in the direction of rotation of the camshaft 18, it isdifficult for the exhaust rocker arm 19 to bear a buckling load.Therefore, it is possible to decrease a load being borne by the exhaustrocker arm 19, so that it is not necessary to implement large-scaledreinforcement to enable the exhaust rocker arm 19 to withstand abuckling load. Accordingly, the exhaust rocker arm 19 can be constructedto be lightweight and compact.

Further, with the above-described unique construction, almost all of aforce with which the cam nose 27 b pushes up the roller bearing 34 canbe effectively used as a force that swings the exhaust rocker arm 19.Thereby, the exhaust rocker arm 19 swings smoothly. Accordingly, it ispossible to make the exhaust rocker arm 19 lightweight and to easilyhandle high-speed rotation of the engine 1.

In addition, since the first rocker shaft 30 is positioned lower thanthe camshaft 18, the upper surface of the cylinder head 3 can be loweredin position. Accordingly, the compactness of the cylinder head 3 isgreatly improved.

The present invention is not limited to the above-described preferredembodiments but can be carried out in various modifications within thescope not departing from the gist of the invention.

While the above-described preferred embodiments are preferably directedto a so-called four-valve engine, in which a pair of exhaust valves anda pair of intake valves are provided in one combustion chamber, thepresent invention is not limited thereto. The present invention can becarried out in, for example, a two-valve engine, in which one exhaustvalve and one intake valve are provided in one combustion chamber, orlikewise in a three-valve engine, in which one exhaust valve and a pairof intake valves are provided in one combustion chamber.

In addition, the rocker arm supported by the first rocker shaft, whichis positioned forward in the direction of rotation of the camshaft, isnot limited to a rocker arm that drives the exhaust valves but may be arocker arm that drives the intake valves.

Further, there is no need to arrange the camshaft on the bore centerline. For example, the camshaft maybe offset toward the exhaust valvesor the intake valves relative to the bore center line.

According to preferred embodiments of the present invention, the loadbeing borne by the first rocker arm is significantly decreased.Accordingly, it is not necessary to undertake large-scaled reinforcementof the first rocker arm to enable the first rocker arm to withstand abuckling load. As a result, the first rocker arm can be made small-sizedand lightweight.

It should be understood that the foregoing description is onlyillustrative of the present invention. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the present invention. Accordingly, the present inventionis intended to embrace all such alternatives, modifications, andvariances that fall within the scope of the appended claims.

1. A valve gear comprising: a camshaft having a first valve gear cam anda second valve gear cam; first and second rocker shafts arranged suchthat the camshaft is disposed between the first and second rockershafts; a first rocker arm swingably supported on the first rocker shaftand having a roller bearing at one end thereof, the roller bearing ofthe first rocker arm being arranged to contact with the first valve gearcam; and a second rocker arm swingably supported on the second rockershaft and having a roller bearing at one end thereof, the roller bearingof the second rocker arm being arranged to contact with the second valvegear cam; wherein the first and second valve gear cams of the camshaft,respectively, include a base circle and a cam nose projecting from thebase circle, and the first rocker arm and the first valve gear cam arearranged such that when the roller bearing of the first rocker armcontacts with the base circle of the first valve gear cam, the firstrocker shaft is located closer to the camshaft than a location of acenter of rotation of the roller bearing of the first rocker arm.
 2. Thevalve gear according to claim 1, wherein the first rocker shaft isarranged to support the first rocker arm and is positioned forwardly ofa center line which passes through a center of the camshaft to extendaxially of a cylinder in a direction of rotation of the camshaft, andthe second rocker shaft is arranged to support the second rocker arm andis positioned rearwardly of the center line in the direction of rotationof the camshaft.
 3. The valve gear according to claim 2, wherein theroller bearing of the first rocker arm and the roller bearing of thesecond rocker arm, respectively, are offset relative to the center lineof the cylinder in an axial direction of the camshaft, and the rollerbearing of the first rocker arm offset farther from the center line thanthe roller bearing of the second rocker arm.
 4. The valve gear accordingto claim 1, wherein the second rocker arm and the second valve gear camare arranged such that when the roller bearing of the second rocker armcontacts with the base circle of the second valve gear cam, the secondrocker shaft is located farther away from the camshaft than a center ofrotation of the roller bearing of the second rocker arm.
 5. The valvegear according to claim 1, wherein the first rocker arm opens and closesat least one exhaust valve and the second rocker arm opens and closes atleast one intake valve.
 6. The valve gear according to claim 1, whereinthe first rocker arm and the first valve gear cam are arranged such thatwhen the roller bearing of the first rocker arm contacts with the basecircle of the first valve gear cam, a force exerted on a contact portionat which the cam nose and the roller bearing of the first rocker armcontact each other does not act along a line which connects a center ofrotation of the roller bearing of the first rocker shaft and a center ofthe first rocker shaft.
 7. The valve gear according to claim 1, whereinthe first rocker arm and the first valve gear cam are arranged such thatthe first rocker shaft is not moved in a direction in which the rollerbearing of the first rocker arm is moved when the cam nose of the firstvalve gear cam contacts and pushes up the roller bearing of the firstrocker shaft.
 8. The valve gear according to claim 1, wherein thefollowing relationship is satisfied: θ1>θ2; wherein θ1 indicates anintersecting angle between a line which connects the center of the firstrocker shaft and the center of rotation of the roller bearing of thefirst rocker arm, and a line which connects a center of rotation of thecamshaft and the center of rotation of the roller bearing of the firstrocker arm, and θ2 indicates an intersecting angle between a line whichconnects a center of the second rocker shaft and a center of rotation ofthe roller bearing of the second rocker arm, and a line which connectsthe center of rotation of the camshaft and the center of rotation of theroller bearing of the second rocker arm.
 9. The valve gear according toclaim 8, wherein the intersecting angle θ1 is larger than approximately90 degrees and the intersecting angle θ2 is smaller than approximately90 degrees.
 10. An engine comprising the valve gear according toclaim
 1. 11. An engine according to claim 10, wherein the engine is afour-cycle engine.
 12. A vehicle comprising the engine according toclaim
 10. 13. A vehicle according to claim 12, wherein the vehicle is amotorcycle.
 14. A valve gear comprising: a camshaft having a first valvegear cam and a second valve gear cam; first and second rocker shaftsarranged such that the camshaft is disposed between the first and secondrocker shafts; a first rocker arm swingably supported on the firstrocker shaft and having a roller bearing at one end thereof, the rollerbearing of the first rocker arm being arranged to contact with the firstvalve gear cam; and a second rocker arm swingably supported on thesecond rocker shaft and having a roller bearing at one end thereof, theroller bearing of the second rocker arm being arranged to contact withthe second valve gear cam; wherein the first and second valve gear camsof the camshaft, respectively, include a base circle and a cam noseprojecting from the base circle, and the first rocker arm and the firstvalve gear cam are arranged such that a relationship between relativepositions of a center of the first rocker shaft, a center of rotation ofthe roller bearing of the first rocker arm, and a center of rotation ofthe camshaft is such that the first rocker arm does not buckle when thecam nose of the first valve gear cam contacts with the roller bearing ofthe first rocker arm to cause the first rocker arm to swing in a valveopening direction.
 15. The valve gear according to claim 14, wherein thefirst rocker shaft is arranged to support the first rocker arm and ispositioned forwardly of a center line which passes through a center ofthe camshaft to extend axially of a cylinder in a direction of rotationof the camshaft, and the second rocker shaft is arranged to support thesecond rocker arm and is positioned rearwardly of the center line in thedirection of rotation of the camshaft.
 16. The valve gear according toclaim 15, wherein the roller bearing of the first rocker arm and theroller bearing of the second rocker arm, respectively, are offsetrelative to the center line of the cylinder in an axial direction of thecamshaft, and the roller bearing of the first rocker arm offset fartherfrom the center line than the roller bearing of the second rocker arm.17. The valve gear according to claim 15, wherein the first rocker armand the first valve gear cam are arranged such that when the rollerbearing of the first rocker arm contacts with the base circle of thefirst valve gear cam, the first rocker shaft is located closer to thecamshaft than a location of a center of rotation of the roller bearingof the first rocker arm.
 18. The valve gear according to claim 17,wherein the second rocker arm and the second valve gear cam are arrangedsuch that when the roller bearing of the second rocker arm contacts withthe base circle of the second valve gear cam, the second rocker shaft islocated farther away from the camshaft than a center of rotation of theroller bearing of the second rocker arm.
 19. The valve gear according toclaim 15, wherein the first rocker arm opens and closes at least oneexhaust valve and the second rocker arm opens and closes at least oneintake valve.
 20. The valve gear according to claim 15, wherein thefirst rocker arm and the first valve gear cam are arranged such thatwhen the roller bearing of the first rocker arm contacts with the basecircle of the first valve gear cam, a force exerted on a contact portionat which the cam nose and the roller bearing of the first rocker armcontact each other does not act along a line which connects a center ofrotation of the roller bearing of the first rocker shaft and a center ofthe first rocker shaft.
 21. The valve gear according to claim 15,wherein the first rocker arm and the first valve gear cam are arrangedsuch that the first rocker shaft is not moved in a direction in whichthe roller bearing of the first rocker arm is moved when the cam nose ofthe first valve gear cam contacts and pushes up the roller bearing ofthe first rocker shaft.
 22. The valve gear according to claim 15,wherein the following relationship is satisfied: θ1>θ2; wherein θ1indicates an intersecting angle between a line which connects the centerof the first rocker shaft and the center of rotation of the rollerbearing of the first rocker arm, and a line which connects a center ofrotation of the camshaft and the center of rotation of the rollerbearing of the first rocker arm, and θ2 indicates an intersecting anglebetween a line which connects a center of the second rocker shaft and acenter of rotation of the roller bearing of the second rocker arm, and aline which connects the center of rotation of the camshaft and thecenter of rotation of the roller bearing of the second rocker arm. 23.The valve gear according to claim 22, wherein the intersecting angle θ1is larger than approximately 90 degrees and the intersecting angle θ2 issmaller than approximately 90 degrees.
 24. An engine comprising thevalve gear according to claim
 15. 25. An engine according to claim 24,wherein the engine is a four-cycle engine.
 26. A vehicle comprising theengine according to claim
 24. 27. A vehicle according to claim 26,wherein the vehicle is a motorcycle.
 28. A valve gear comprising: acamshaft having a first valve gear cam and a second valve gear cam;first and second rocker shafts arranged such that the camshaft isdisposed between the first and second rocker shafts; a first rocker armswingably supported on the first rocker shaft and having a rollerbearing at one end thereof, the roller bearing of the first rocker armbeing arranged to contact with the first valve gear cam; and a secondrocker arm swingably supported on the second rocker shaft and having aroller bearing at one end thereof, the roller bearing of the secondrocker arm being arranged to contact with the second valve gear cam;wherein the first and second valve gear cams of the camshaft,respectively, include a base circle and a cam nose projecting from thebase circle, and the first rocker arm and the first valve gear cam arearranged such that when the roller bearing of the first rocker armcontacts with the base circle of the first valve gear cam, a forceexerted on a contact portion at which the cam nose and the rollerbearing of the first rocker arm contact each other does not act along aline which connects a center of rotation of the roller bearing of thefirst rocker shaft and a center of the first rocker shaft.
 29. The valvegear according to claim 28, wherein the first rocker shaft is arrangedto support the first rocker arm and is positioned forwardly of a centerline which passes through a center of the camshaft to extend axially ofa cylinder in a direction of rotation of the camshaft, and the secondrocker shaft is arranged to support the second rocker arm and ispositioned rearwardly of the center line in the direction of rotation ofthe camshaft.
 30. The valve gear according to claim 29, wherein theroller bearing of the first rocker arm and the roller bearing of thesecond rocker arm, respectively, are offset relative to the center lineof the cylinder in an axial direction of the camshaft, and the rollerbearing of the first rocker arm offset farther from the center line thanthe roller bearing of the second rocker arm.
 31. The valve gearaccording to claim 28, wherein the second rocker arm and the secondvalve gear cam are arranged such that when the roller bearing of thesecond rocker arm contacts with the base circle of the second valve gearcam, the second rocker shaft is located farther away from the camshaftthan a center of rotation of the roller bearing of the second rockerarm.
 32. The valve gear according to claim 28, wherein the first rockerarm opens and closes at least one exhaust valve and the second rockerarm opens and closes at least one intake valve.
 33. The valve gearaccording to claim 28, wherein the first rocker arm and the first valvegear cam are arranged such that the first rocker shaft is not moved in adirection in which the roller bearing of the first rocker arm is movedwhen the cam nose of the first valve gear cam contacts and pushes up theroller bearing of the first rocker shaft.
 34. The valve gear accordingto claim 28, wherein the following relationship is satisfied: θ1>θ2;wherein θ1 indicates an intersecting angle between a line which connectsthe center of the first rocker shaft and the center of rotation of theroller bearing of the first rocker arm, and a line which connects acenter of rotation of the camshaft and the center of rotation of theroller bearing of the first rocker arm, and θ2 indicates an intersectingangle between a line which connects a center of the second rocker shaftand a center of rotation of the roller bearing of the second rocker arm,and a line which connects the center of rotation of the camshaft and thecenter of rotation of the roller bearing of the second rocker arm. 35.The valve gear according to claim 34, wherein the intersecting angle θ1is larger than approximately 90 degrees and the intersecting angle θ2 issmaller than approximately 90 degrees.
 36. An engine comprising thevalve gear according to claim
 28. 37. An engine according to claim 36,wherein the engine is a four-cycle engine.
 38. A vehicle comprising theengine according to claim
 36. 39. A vehicle according to claim 38,wherein the vehicle is a motorcycle.
 40. A valve gear comprising: acamshaft having a first valve gear cam and a second valve gear cam;first and second rocker shafts arranged such that the camshaft isdisposed between the first and second rocker shafts; a first rocker armswingably supported on the first rocker shaft and having a rollerbearing at one end thereof, the roller bearing of the first rocker armbeing arranged to contact with the first valve gear cam; and a secondrocker arm swingably supported on the second rocker shaft and having aroller bearing at one end thereof, the roller bearing of the secondrocker arm being arranged to contact with the second valve gear cam;wherein the first and second valve gear cams of the camshaft,respectively, include a base circle and a cam nose projecting from thebase circle, and the first rocker arm and the first valve gear cam arearranged such that the first rocker shaft is not moved in a direction inwhich the roller bearing of the first rocker arm is moved when the camnose of the first valve gear cam contacts and pushes up the rollerbearing of the first rocker shaft; θ1 indicates an intersecting anglebetween a line which connects the center of the first rocker shaft andthe center of rotation of the roller bearing of the first rocker arm,and a line which connects a center of rotation of the camshaft and thecenter of rotation of the roller bearing of the first rocker arm: and θ1is larger than approximately 90 degrees.
 41. The valve gear according toclaim 40, wherein the first rocker shaft is arranged to support thefirst rocker arm and is positioned forwardly of a center line whichpasses through a center of the camshaft to extend axially of a cylinderin a direction of rotation of the camshaft, and the second rocker shaftis arranged to support the second rocker arm and is positionedrearwardly of the center line in the direction of rotation of thecamshaft.
 42. The valve gear according to claim 41, wherein the rollerbearing of the first rocker arm and the roller bearing of the secondrocker arm, respectively, are offset relative to the center line of thecylinder in an axial direction of the camshaft, and the roller bearingof the first rocker arm offset farther from the center line than theroller bearing of the second rocker arm.
 43. The valve gear according toclaim 40, wherein the second rocker arm and the second valve gear camare arranged such that when the roller bearing of the second rocker armcontacts with the base circle of the second valve gear cam, the secondrocker shaft is located farther away from the camshaft than a center ofrotation of the roller bearing of the second rocker arm.
 44. The valvegear according to claim 40, wherein the first rocker arm opens andcloses at least one exhaust valve and the second rocker arm opens andcloses at least one intake valve.
 45. The valve gear according to claim40, wherein the following relationship is satisfied: θ1>θ2; wherein θ2indicates an intersecting angle between a line which connects a centerof the second rocker shaft and a center of rotation of the rollerbearing of the second rocker arm, and a line which connects the centerof rotation of the camshaft and the center of rotation of the rollerbearing of the second rocker arm.
 46. The valve gear according to claim45, wherein the intersecting angle θ2 is smaller than approximately 90degrees.
 47. An engine comprising the valve gear according to claim 40.48. An engine according to claim 47, wherein the engine is a four-cycleengine.
 49. A vehicle comprising the engine according to claim
 47. 50. Avehicle according to claim 49, wherein the vehicle is a motorcycle. 51.A four-cycle engine comprising: a cylinder having a bore center line; acylinder head connected to the cylinder and having an exhaust valve andan intake valve; a camshaft supported by the cylinder head and having afirst valve gear cam and a second valve gear cam; first and secondrocker shafts arranged such that the camshaft is disposed between thefirst and second rocker shafts; a first rocker arm swingably supportedon the first rocker shaft and having a roller bearing at one endthereof, the roller bearing of the first rocker arm being arranged tocontact with the first valve gear cam, the first rocker arm acting todrive one of the exhaust valve and the intake valve; and a second rockerarm swingably supported on the second rocker shaft and having a rollerbearing at one end thereof, the roller bearing of the second rocker armbeing arranged to contact with the second valve gear cam, the secondrocker arm acting to drive the other of the exhaust valve and the intakevalve; wherein the first and second valve gear cams of the camshaft,respectively, include a base circle and a cam nose projecting from thebase circle, and the first rocker arm and the first valve gear cam arearranged such that when the roller bearing of the first rocker armcontacts with the base circle of the first valve gear cam, the firstrocker shaft is located closer to the camshaft than a location of acenter of rotation of the roller bearing of the first rocker arm. 52.The four-cycle engine according to claim 51, wherein the exhaust valveand the intake valve, respectively, include a valve stem at a first endthereof, and the first rocker arm and the second rocker arm,respectively, include another end to push the valve stem, and thecylinder head includes a first opening to expose abutting portions ofthe other end of the first rocker arm and the valve stem, and a secondopening to expose abutting portions of the other end of the secondrocker arm and the valve stem, the first opening and the second openingbeing arranged to be opposed to each other with the bore center linetherebetween and covered by respective common removable covers, and thecovers include first and second walls on inner surfaces thereof whichare opposed to the abutting portions and arranged to receive alubricating oil, the first wall being formed with supply ports throughwhich the lubricating oil is applied to the abutting portions of theother end of the first rocker arm and the valve stem, and the secondwall being formed with supply ports through which the lubricating oil isapplied to the abutting portions of the other end of the second rockerarm and the valve stem.